A reader at the Math 1 level has enough mathematical ability to encompass "good at math" in a colloquial sense. They know at least some basic algebra, and how to apply algebraic thinking to problems in some contexts. If you threw a simple to moderate math puzzle at them, they could probably figure out what they were supposed to do without balking and saying "nope, I don't know how to do this".

Writing for a Math 1 audience

At this level, you can start to use letters to represent numbers as variables and manipulate them directly.